Three-dimensional, ultrasonic transducer arrays, methods of making ultrasonic transducer arrays, and devices including ultrasonic transducer arrays

ABSTRACT

Systems, apparatus, and associated methods of forming the systems and/or apparatus may include imaging devices that may comprise multiple arrays of ultrasonic transducer elements for use in a variety of applications. These multiple arrays of ultrasonic transducer elements can be arranged to form a three-dimensional imaging device. Non-coplanar arrays of ultrasonic transducer elements can be coupled together. These imaging devices may be used as medical imaging devices. Additional apparatus, systems, and methods are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/685,199, filed on Mar. 12, 2007, which application claims the benefitof priority of U.S. provisional application No. 60/780,828, entitled“Wireless Multi-directional Capsule Endoscopes,” filed on Mar. 10, 2006,U.S. provisional patent application No. 60/804,018, entitled“Multi-directional Ultrasonic Imager Array,” filed on Jun. 6, 2006, andU.S. provisional application No. 60/836,162, entitled “Monolithic ThreeDimensional Ultrasonic Transducer Array with Through-Wafer ElectricalInterconnects,” filed on Aug. 7, 2006, the contents of all of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention is directed generally to transducer arrays,methods of making transducer arrays, and devices including transducerarrays. More particularly, the present invention is directed tothree-dimensional, ultrasonic transducer arrays, methods of makingultrasonic transducer arrays, and medical devices including ultrasonictransducer arrays.

BACKGROUND

Some conventional ultrasonic transducers, either piezoelectric orCapacitive Micromachined Ultrasonic Transducer (CMUT), are built on abulky (e.g., greater than 500 μm thick) piece of silicon or othersubstrate and arranged in one-dimensional or two-dimensional arrays formedical imaging. Many of these conventional ultrasonic imagers are ableto look in only one direction.

One exception to the above-described conventional transducers is thepiezoelectric side-viewing imager used in Intravascular UltrasonicImagers (IVUS). This device includes a one-dimensional array ofpiezoelectric transducer mounted on the surface of a catheter to form acylindrical array that can scan 360°. However, this piezoelectrictransducer is limited to providing only two-dimensional images ratherthan real-time three-dimensional images as needed by many diagnosticprocesses.

Additionally, due to the difficulty of mounting multiple pieces oftransducers on the front and the side of a catheter platform, commercialIVUS are typically equipped with either a side-looking imager or afront-looking imager, but not both.

Thus, it may be desirable to provide a miniature, monolithic ultrasonicimager having multi-direction-looking capabilities and the ability toprovide real-time three-dimensional images.

Currently there is a commercially available ingestible capsuleendoscope, called Pillcam, which uses a miniature CMOS camera hidinginside a plastic capsule to shoot color photographs of thegastrointestinal (GI) tract. As a minimally invasive device, Pillcam canbe swallowed like a vitamin pill and can provide useful imageinformation for diagnosing stomach or small-intestine disorders.Although Pillcam causes far less discomfort for the patient and canreach a much further extent than is capable by the traditional tubeendoscope, it has three basic constraints. First, it can only see thesurface of the gastrointestinal tract and can not see into the tissue.This limits its capability in determining the extent of a tumor orabnormality in the digestive organs. Secondly, Pillcam cannot shootimages of the colon or rectum because of blockage of stool. As a result,a colonscopy is need for diagnosing the disorders in the colon or rectumand the process is very discomfortable. Thirdly, Pillcam has only onecamera and can only look at one direction when it (randomly) travelsthrough the digestive tract. This could result in missing of criticalimages for a diagnosing process.

It may be desirable to provide an ingestible capsule endoscope that canprovide imaging in more than one direction and/or an endoscope that canprovide ultrasonic and visible light imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side perspective view of an exemplary monolithic,multi-directional ultrasonic transducer array in accordance with variousaspects of the disclosure.

FIG. 2 is a plan view of the transducer array of FIG. 1 in anunassembled configuration.

FIG. 3 illustrates a side perspective view of another exemplarymonolithic, multi-directional. ultrasonic transducer array in accordancewith various aspects of the disclosure.

FIG. 4 is a side perspective view of an exemplary wireless,multi-directional ultrasonic capsule endoscope in accordance withvarious aspects of the disclosure.

FIGS. 5A-5R are cross-sectional views of an exemplary transducerillustrating an exemplary process in accordance with aspects of theinvention.

FIGS. 6A-6BB are cross-sectional views of an exemplary transducerillustrating an exemplary process in accordance with aspects of theinvention.

DETAILED DESCRIPTION

In various aspects, the present disclosure is directed to medicalimaging devices that may comprise an array of ultrasonic transducerelements. Each transducer element may comprise a substrate having adoped surface creating a highly conducting surface layer, a layer ofthermal oxide on the substrate, a layer of silicon nitride on the layerof thermal oxide, a layer of silicon dioxide on the layer of siliconnitride, and a layer of conducting thin film on the layer of silicondioxide. The layers of silicon dioxide and thermal oxide may sandwichthe layer of silicon nitride, and the layer of conducting thin film maybe separated from the layer of silicon nitride by the layer of silicondioxide.

In some aspects, methods of fabricating an array of ultrasonictransducers for a medical imaging device may comprise doping a surfaceof a substrate to create a highly conducting surface layer, growing alayer of thermal oxide on the highly conducting surface layer of thesubstrate, depositing a layer of silicon nitride on the layer of thermaloxide, depositing a layer of silicon dioxide on the layer of siliconnitride, and depositing a layer of conducting thin film on the layer ofsilicon dioxide. The layers of silicon dioxide and thermal oxide maysandwich the layer of silicon nitride, and the layer of conducting thinfilm may be separated from the layer of silicon nitride by the layer ofsilicon dioxide

In various aspects, the present disclosure is directed to medicalimaging devices comprising an array of ultrasonic transducer elements.Each transducer element may comprise a substrate having a doped surfacecreating a highly conducting surface layer, a plurality of sandwicheddielectric layers, a first layer of conducting thin film on thedielectric layers, and a second layer of conducting thin film. The firstlayer of conducting thin film and the substrate may sandwich thedielectric layers. The second layer of conducting thin film may have aportion in contact with the first layer of conducting thin film and aportion separated from the first layer of conducting thin film, saidsecond layer having at least one hole therethrough. A vacuum cavity maybe between the first and second layers of conducting thin film. Eachelement may include a silicon nitride film on the second layer ofconducting thin film, with the silicon nitride film extending throughthe hole in the second layer and into the vacuum cavity so as to preventportions of the second layer from collapsing the vacuum cavity andcontacting the first layer.

In some aspects, methods of fabricating a medical imaging device maycomprise doping a surface of a substrate to create a highly conductingsurface layer, growing a layer of thermal oxide on the highly conductingsurface layer of the substrate, depositing at least one layer ofdielectric film on the layer of thermal oxide, and depositing a firstlayer of conducting thin film on the at least one layer of dielectricfilm. The first layer of conducting thin film and the substrate maysandwich the dielectric layers. The methods may include depositing andpatterning a sacrificial oxide on the first layer and depositing asecond layer of conducting thin film on the patterned sacrificial oxidesuch that the second layer may have a portion in contact with the firstlayer of conducting thin film and a portion separated from the firstlayer of conducting thin film. The methods may include creating at leastone hole through the second layer, wherein the hole extends into thesacrificial oxide. The methods may include depositing a silicon nitridefilm on the second layer of conducting thin film, wherein the siliconnitride film extends through the hole in the second layer and into thehole in the sacrificial oxide. The methods may include removing thesacrificial oxide to create a vacuum cavity between the first and secondlayers of conducting thin film.

In various aspects, the present disclosure is directed to medicaldevices configured to be swallowed by a patient. The devices may includea plurality of micro-machined transducers. Each transducer may beconfigured to send and receive ultrasonic waves in a direction differentfrom that of other transducers.

In various aspects, wireless capsule endoscopes may comprise a pluralityof micro-machined transducers. Each transducer may be configured to sendand receive ultrasonic waves in a direction different from that of othertransducers.

In some aspects, the present disclosure is directed to methods ofimaging a digestive tract of a patient. The methods may compriseintroducing a capsule endoscope into the digestive tract, wherein thecapsule contains a plurality of micro-machined transducers configured tosend and receive ultrasonic waves in a direction different from that ofother transducers. The method may also include generating ultrasonicwaves with said plurality of transducers, receiving ultrasonic wavesbeing directed toward said transducers by portions of the digestivetract, and generating ultrasonic images based on the received waves.

An exemplary embodiment of a monolithic multi-directional ultrasonictransducer array 100 is illustrated in FIGS. 1 and 2. According to someaspects, the transducer array 100 may comprise an imager array. Thetransducer array 100 may include integrated micro-machined (e.g., MEMS)ultrasonic transducers 800, 900 and front-end CMOS signal processingcircuitry 120 on one piece of silicon substrate 850, 950. As a result ofthe monolithic structure, a better signal-to-noise ratio and thereforebetter image quality may be achieved by the Imager array.

The exemplary monolithic ultrasonic imager array 100 may be divided intopieces of ultrasound-elements/CMOS-circuitry plates between which thin(1-20 μm thick) flexible silicon membranes are used for interconnection.These small plates, each typically measuring, for example, 0.8millimeter×0.8 millimeter (for a 20 MHz ultrasonic imager) and 40 μm-100μm thick, can be folded into a three-dimensional prism structure asshown in FIG. 1. The thin flexible silicon membranes may not onlyprovide the physical connection for device plates but also support thethin-film electrical interconnects between the imager devices and thefront-end CMOS circuitry. This 3-D ultrasonic imager array can look atmultiple directions as needed for many medical applications includingcapsule endoscope and intravascular diagnosis. Although in FIGS. 1 and 2a hexagonal prism is illustrated, this technology is capable ofconstructing multi-direction looking ultrasonic cameras of any prismshape or a cylindrical shape, as shown in FIG. 3.

Referring now to FIG. 4, an ultrasonic capsule camera 1000 isillustrated. On the capsule camera 1000, each ultrasonic imagercomprises a two-dimensional ultrasonic transducer array. The capacitiveultrasonic transducer element is able to generate ultrasonic waves uponapplication of an a.c. electrical signal on its membrane. The samemembrane may also work as an ultrasonic sensor, which will deform andgenerate an electrical signal upon reception of impinging ultrasonicwaves.

After being assembled, miniature fixing platforms made of micromachinedsilicon may be used to fix the ultrasonic imager in its prism orcylindrical shape, and silicone or other bio-compatible polymers may beused to fill into the cavity inside the prism so the whole structurewill be glued together. The polymer may also provide the mechanicalsupport for this pill camera so it would not crush easily by externalpressure. The diameter of this ultrasonic camera pill typically rangesfrom 0.4 millimeters (for 50 MHZ ultrasounds) to 1.6 millimeters (for 10MHz ultrasounds), while its length is about 2 millimeters. A MEMS spiralinductor antenna may be integrated with this pill structure eithermonolithically or flip-chip bonded on the silicon substrate for sendingout the image signals. This multi-direction-looking ultrasonic cameramay be complementary to an optical imager capsule endoscope (i.e.,Pillcam) and can provide additional diagnostic information not availablefrom a regular optical camera capsule endoscope. This exemplaryultrasonic imaging pill may be integrated with a CMOS imager, such thatthis minimally invasive capsule is capable of shooting ultrasound imagesin addition to color photographs from inside the digestive organs

The exemplary ultrasonic capsule camera is able to look at multipledirections simultaneously when traveling through the digestive tract.Additionally, ultrasound can penetrate through the tissue and providein-depth images of tumors or any abnormity of the digestive organs. Thiscapability may be useful for diagnosing the early stage of cancer tumorsand other tissue disorders. On this platform, ultrasound transducers ofdifferent resonant frequency can be integrated on one imager fordifferent depth detection/imaging. Due to the penetration capability,this ultrasonic imaging capsule is capable of grabbing images of thecolon or rectum when it travels inside them. This could potentiallyprovide an alternative for colonscopy, which is an expensive diagnosticprocess and generally causes much discomfort for the patient.

In addition to capsule endoscopy, this multi-direction-lookingultrasonic device may be useful for intravascular ultrasonic (IVUS)imaging. Most of the commercial IVUS heads are made by assemblingpiezoelectric ultrasound transducers that are bulky and difficult tointegrate with signal processing circuitry. For example, the commercialIVUS heads are typically 0.5-4 millimeter in diameter. In contrast, theMEMS monolithic imaging head described in this invention disclosure isabout 3-4 times smaller in diameter than commercial piezoelectric IVUStools and are suitable for reaching inside fine vessels. The MEMSultrasonic transducer devices on this imaging system are made ofthin-film drum structures. Drums of different diameter (whichcorresponds to the target frequency of the ultrasound) can bemonolithically integrated on one silicon substrate to emit and senseultrasounds of different wavelength. As a result, one more advantage ofusing the exemplary MEMS ultrasonic imager device of this disclosure isthat the front-looking imagers can be designed independently of theside-looking imagers in their imaging wavelength/frequency. Forconventional ceramic materials, it is hard to do so since a single pieceof piezoelectric element is cut, and the inter-element spacing must beidentical. Allowing front-looking and side-looking imager elements to“see” at different wavelengths will eliminate many limitationsassociated with the current IVUS and open new applications incardiovascular diagnosis. Additionally, all of the conventional IVUS usea one-dimensional ultrasonic transducer array for side looking, and theyprovide only two-dimensional images perpendicular to the orientation ofthe blood vessel. The MEMS ultrasonic transducer arrays of thisdisclosure integrate a two-dimensional array along the catheter sidewalldirection and are uniquely the first IVUS heads capable of real-timethree-dimensional images for side viewing. Thus, the transducer arraysof this disclosure can provide diagnostic information not available fromany conventional IVUS.

On this ultrasonic capsule camera, each ultrasonic imager comprises atwo-dimensional ultrasonic transducer array 800, 900. The capacitiveultrasonic transducer element is able to generate ultrasonic waves uponapplication of an a.c. electrical signal on its membrane. The samemembrane may also work as an ultrasonic sensor, which will deform andgenerate an electrical signal upon reception of impinging ultrasonicwaves.

The endoscope may be similar to the Micro-Electro-Mechanical System(MEMS) implantable ultrasonic imager array, described for example, inU.S. provisional patent application No. 60/734,385 and U.S. patentapplication Ser. No. 11/320,921, the contents of both of which areincorporated herein by reference in their entirety. Different from thelimited one-direction looking capability of Pillcam, the exemplarywireless multi-direction-looking ultrasonic capsule endoscope 1000 shownin FIG. 4 is able to provide front-looking and/or multi-directionalside-looking capabilities simultaneously.

Referring now to FIGS. 5A-5R, an exemplary process of making anexemplary transducer 800 will be described. As shown in FIG. 5A, thesurface of the silicon substrate 850 may be doped using, for example,diffusion or ion implantation to create a highly conducting surfacelayer 851. This layer 851 may reduce or prevent charge feedthrough tothe substrate 850 from the electrostatic devices on the surface.

After the doping process, the layer of thermal oxide 852 may be grown onthe surface of the substrate 850 and serve as a first dielectric layer.Two additional dielectric layers, for example, the silicon nitride layer854 and the silicon dioxide layer 856 may then be deposited on top ofthe thermal oxide layer 852 using, for example, low pressure chemicalvapor deposition (LPCVD) or other known chemical vapor deposition (CVD)processes.

Referring now to FIG. 5B, on top of the sandwiched dielectric layers(e.g., silicon dioxide/silicon nitride/silicon dioxide), thesemiconductor layer 858 comprising, for example, polysilicon or otherconducting thin film, may be deposited, doped, and annealed to reducethe residual stress. The semiconductor layer 858 may work as a counterelectrode for the drum membrane as well as for the electricalinterconnects.

As shown in FIG. 5C, a photolithography process may be used to definepatterns of the semiconductor layer 858. A dry etching, for example,hydrogen fluoride (HF) etching, may be used to remove the exposedportion (not covered by a masking photoresist) of the semiconductorlayer 858, thus transferring the patterns into the semiconductor layer858. The process may then proceed to the step described in relation toFIG. 5D. However, in an alternative embodiment (not shown), with themasking photoresist still on, another dry etching, for example, HFetching, may be used to remove the exposed top dielectric layer 856comprising, for example, silicon dioxide. Thus, the second dielectriclayer 854 comprising, for example, silicon nitride, may be exposedeverywhere except where the semiconductor layer 858 is still present.The top dielectric layer 856 is thus self-aligned with the semiconductorlayer 858 comprising, for example, a polysilicon structure. With thisarrangement, the semiconductor layer 858 may be anchored to thesubstrate 850 through the third dielectric layer 856, which may comprisea silicon dioxide film instead of silicon nitride. However, theremainder of the substrate 850 is covered by the second dielectric layer854, which may comprise, for example, silicon nitride, that survivesfrom HF etching in the subsequent HF release etching process.

Referring to FIG. 5D, a thin layer of a sacrificial oxide 860 may bedeposited next. The thickness of this sacrificial oxide 860 determinesthe gap height between the membrane and its counter electrode. As shownin FIG. 5E, the sacrificial oxide may be patterned using aphotolithography process and a dry etching such as, for example, HFetching, to form dimples (not shown) and anchoring holes.

Turning now to FIG. 5F, a second semiconductor layer 862 comprising, forexample, structural polysilicon, may be deposited, doped, and annealed.As shown in FIG. 5G, a thin layer 864 of silicon dioxide may bedeposited, for example, via LPCVD. The thin layer 864 of silicon dioxidemay be about 500 angstroms thick.

Next, as shown in FIG. 5H, the thin layer of silicon dioxide 864 and thesecond semiconductor layer 862 may be patterned to form a membrane usinga photolithography process and a dry etching such as, for example, HFetching. The etching process may etch through the second semiconductorlayer and overetch into the sacrificial oxide 860 to form pits 865 intothe sacrificial oxide. The pits may be about 300-500 angstroms deep.

Referring now to FIG. 51, a layer 866 of silicon nitride may bedeposited, for example, via LPCVD. The thin layer 864 of silicon dioxidevertically separates the second semiconductor layer 862 from the layer866 of silicon nitride. The layer 866 of silicon nitride may be about0.3 μm and may fill the dimple pits in the sacrificial oxide 860. Thetypical area of the pits is 2 μm×2 μm, and the thin film layer 866 ofsilicon nitride may not be able to fill the pits completely. Ahollow-core nitride column may thus be formed.

As shown in FIG. 5J, the thin layer of silicon nitride film 866 may bepatterned via a photolithography process and a dry etching such as, forexample, HF etching. Then, as shown in FIG. 5K, conventional dry or wetetching may be used to remove the sacrificial oxide 860 and the topdielectric layer 856 (if not removed in the step shown in FIG. 5C) torelease microstructures of the second semiconductor layer 862 from thesubstrate 850. This dry/wet etching may also undercut the thin oxidelayer under the first semiconductor layer 858. Due to the etch ratedifference between the sacrificial oxide and the high-quality LPCVDoxide under the first semiconductor layer 858, the length of theundercut is small and will not degrade the anchoring robustness of thefirst semiconductor layer 858.

Turning to FIG. 5L, a photolithography step and a dry etching may beused to remove regions of the layer 854 of silicon nitride over whichmetal interconnects will meander. Removal of these regions of thesilicon nitride layer 854 reduces the contact area between the metallines and the nitride film 854 in order to minimize charging problems.

Referring now to FIG. 5M, a thick layer of PECVD or other depositedoxide 868, for example, tetraethoxysilane (TEOS), may be used to sealthe release holes. As these thin-film deposition processes are performedin vacuum, the cavity 869 under the semiconductor membrane 862 may besealed under vacuum. A photolithography process and a dry and/or wetetch may be used to pattern the sealing oxide 868 such that the oxidethickness is reduced to about 4000 angstroms on most of the device areasexcept areas around the release holes and in the center of the membrane,as shown in FIG. 5N. The oxide left on the center of the membrane may beused to improve the frequency response of the membrane.

Referring now to FIG. 5O, a metal layer may be deposited and patternedto form metal interconnects 880. Turning to FIG. 5P, a polymerpassivation dielectric layer 882 may be deposited and patterned. Thepassivation layer may include a layer of PECVD oxide and/or a relativelythick (e.g., several microns) layer of parylene C.

As shown in FIG. 5Q, conventional dry or wet etching from the backside884 may be used to thin the silicon substrate 850, for example, toapproximately 80-140 μm thick. Turning to FIG. 5R, a photolithographystep and a dry etching from the backside 884 may be used to form deeptrenches 886 in the substrate. The transducer array 800 resulting fromthe process illustrated via FIGS. 9A-9R may be used is a variety ofmedical devices, as discussed in more detail below.

Referring now to FIGS. 6A-6Z, an exemplary process of making anexemplary transducer 900 will be described. The fabrication of thismonolithic three-dimensional ultrasonic array 900 starts with a thicksilicon wafer approximately 100 μm thick. As shown in FIG. 6A, layers ofsilicon dioxide 951, 1951 may be grown thermally on both surfaces 902,1902 of the substrate 950 and serve as a first dielectric layer. Thelayers 951, 1951 of silicon dioxide may be relatively thick at about 1.0μm. Dielectric layers, for example, silicon nitride layers 952, 1952 maythen be deposited on top of the thermal oxide layers 951, 1951 using,for example, low pressure chemical vapor deposition (LPCVD) or otherknown chemical vapor deposition (CVD) processes. These nitride layersare used to anchor the micro-structures to the silicon substrate duringthe HF sacrificial etching process. The silicon nitride layers 952, 1952may be approximately 2000 angstroms thick.

Referring now to FIG. 6B, a photolithography process and a reactive ionetching are next used to pattern the backside of the substrate, exposingthe holes for through-wafer channels. A deep silicon dry etching is thenused to create an array of through-wafer channels 1953. The dimensionsof the channel openings may be, for example, 4 μm×4 μm. As shown in FIG.6C, thermal oxidation is next used to convert the surface of the channelwall from silicon into silicon dioxide. Thus, the sidewall of thethrough-wafer channels is covered with a layer of silicon dioxide 1951.This oxide layer is used to insulate the substrate from the conductorthat is to be filled in the channel in the subsequent step.

Turning to FIG. 6D, a thin (approx. 250-300 angstroms) layer of silicondioxide 954, 1954 is then deposited on both sides of the substrate viaLPCVD deposition. This thin silicon dioxide layer covers the siliconnitride layer. This oxide layer is used to separate the dopedpolysilicon (to be deposited next) from the silicon nitride for reducingcharging problems associated with silicon nitride film.

A layer of in-situ doped polysilicon 956, 1956 is next deposited on bothsides of the substrate using LPCVD, as shown in FIG. 6E. Thispolysilicon paves the surface of the substrate and forms a continuousconduction channel from the backside of the wafer to the front end ofthe channel. Next, as illustrated in FIG. 6F, the polysilicon on thefront-side of the wafer is next patterned to form the polysilcionelectrodes and anchoring structures. The patterning may be achieved, forexample, via photolithography and a reactive ion etching.

Referring now to FIG. 6G, a layer of CVD oxide 958, 1958 is nextdeposited. This layer of CVD oxide 958, 1958 serves as a sacrificiallayer. A photolithography and a reactive ion etching are then used toopen anchor holes 959 in this sacrificial oxide, as shown in FIG. 6H. Asillustrated in FIG. 6I, another photolithography and a dry etching arenext used to open via holes 961 over the through-wafer channels. The viaholes 961 cut through multiple layers of oxide and nitride and exposethe polysilicon in the through-wafer channel.

Turning to FIG. 6J, a layer of polysilicon 960, 1960 is deposited viaLPCVD deposition and doped. This polysilicon fills in the via-holes toform a continuous conduction path through the through-wafer channel tothe backside polysilicon. It also works as the structural polysiliconfor constructing the transducer membrane and other microstructures.

Next, as shown in FIG. 6K, a thin (200-300 angstroms) layer 962, 1962 ofoxide, for example, silicon dioxide, is deposited using LPCVD. Thisoxide is used to separate the doped polysilicon from a silicon nitridethat is to be deposited subsequently. A photolithography and a reactiveion etching are then used to pattern the thin buffer oxide and thestructural polysilicon using one mask, as illustrated in FIG. 6L. Thispatterning process defines the geometrical shape of the transducermembrane and excavates an array of holes 963 inside the membrane whichwill be used to form the dielectric posts. In this dry etching process,after the exposed polysilicon is completely etched away, an intentionalover-etch is used to create an array of 200-300 angstroms deep pits onthe sacrificial oxide. These pits are to be used as molds to formdielectric posts which protrude from the lower surface of thepolysilicon membrane after a subsequent nitride deposition. The functionof the posts is to prevent shorting of the polysilicon membrane to itscounter electrode.

Referring now to FIG. 6M, a layer of LPCVD silicon nitride 964, 1964 isnext deposited to fill in the post holes as well as coat the rest of thewafer surface. As shown in FIG. 6N, the wafer is then patterned using aphotolithography process on the front side. Only the silicon nitridefilm covering the post holes or around their vicinity is reserved. Therest is etched away using, for example, reactive ion etching. Thisetching step also partially etches the thin oxide layer under thesilicon nitride.

The next step is the removal of sacrificial oxide 958 to free thepolysilicon microstructures, as illustrated in FIG. 6O. The sacrificialoxide 958 may be etched away using either a wet or dry etching. In thisrelease etching process, the thin oxide 954 under the polysilicon 956 ispartially undercut forming cavities 965, which may be sealed undervacuum. However, due to the very thin oxide used in this kind ofsandwiched (nitride/thin-oxide/polysilicon) structure and the slow etchrate of the high-quality LPCVD oxide (compared with the fast etch rateof sacrificial oxide), the extent of undercut is limited to anacceptable range and will not degrade the mechanical strength of theanchors of the microstructures to the substrate.

Turning to FIG. 6P, a sealing oxide 966, 1966, typically TEOS depositedby PECVD or LPCVD, is next deposited to seal the release holes. Thissealing oxide is patterned using photolithography and a dry and/or wetetching, as shown in FIG. 6Q. The sealing oxide is stripped from thebackside using wet or dry etching.

Referring to FIG. 6R, with the front-side of the wafer protected by alayer of photoresist, another dry or wet etching is used to remove thesilicon nitride 1964, the silicon dioxide 1962, the polysilicon 1960,and the sacrificial oxide 1958 from the backside. A photolithographyprocess and a dry etching, for example, reactive ion etching, are thenused to pattern the polysilicon on the backside, as shown in FIG. 6S.Both sides of the wafer are next coated with a layer of polymer 968,1968, as illustrated by FIG. 6T. As shown in FIG. 6U, the polymer on thebackside is patterned to expose the via holes 1969 on which metalinterconnects will connect to the polysilicon underneath. The patterningmay be achieved, for example, by photolithography and reactive ionetching.

Turning now to FIG. 6V, a thin layer of metal 1970 is next deposited andpatterned as electrical interconnects on the backside. Another layer ofpolymer 1972 is then deposited on the backside 1902 to passivate themetal interconnects, as shown in FIG. 6W. This polymer film ispatterned, for example, by photolithography and reactive ion etching, toexpose the metal on the bonding pad areas 1973 on the backside, asillustrated in FIG. 6X.

Referring now to FIG. 6Y, a photolithography process and a dry etching,for example, reactive ion etching, are next used to pattern the backsidedielectrics (FIG. 28). This step exposes the silicon substrate over auniform band area surrounding the device boundary. This is the firstmasking step to define the boundary for individual devices. Anotherphotolithography process and a dry etching are next used to pattern thedielectrics on the front side, as shown in FIG. 6Z. This step exposesthe silicon substrate on areas for inter-imager-plate connection as wellas the boundary of the device. This is the second masking step to definethe boundary for individual device separation.

With the same masking photoresist used for the processing step describedin connection with FIG. 6Z, a deep silicon etching is used to cutthrough the wafer. As illustrated in FIG. 6AA, this etching stepreleases the imager array from the substrate. Referring to FIG. 6BB,this etching step also forms a flexible dielectric inter-plateconnection 1974 between imager plates by removing the substrate siliconon the connection region. The imager array can then be assembled into athree dimensional imaging device as shown in FIGS. 1-4.

Referring now to FIG. 7, an exemplary medical device 1000 may compriseone of the aforementioned transducer arrays. The exemplary medicaldevice 1000 may work both as an emitter to generate ultrasounds and asensor to detect ultrasounds. When the device 1000 works as an emitter,the transducer 818 is driven by an ac electrical signal applied betweenthe membrane 862 and the counter electrode 860. The ac electrical signalmay produce a time varying electrostatic force on the membrane 862 thaturges the membrane 862 to move up and down. This movement generatesmechanical waves which transmit out to the media surrounding themembrane 862. During this electrostatic actuation process, electricalcharges are periodically received into and removed from the variablecapacitor, which is defined by the membrane 862 and the counterelectrode 860.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the medical devices andmethods of the present invention without departing from the scope of theinvention. Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only.

What is claimed is:
 1. An imaging device comprising: a first array ofultrasonic transducer elements; a second array of ultrasonic transducerelements; and a flexible dielectric that couples the first array to thesecond array such that the first array and the second array arenon-coplanar and arranged as part of a three-dimensional structure. 2.The imaging device of claim 1, wherein the first array and the secondarray are arranged as part of a multi-sided prism with a number of sidesgreater than or equal to six.
 3. The imaging device of claim 2, whereinthe multi-sided prism is a hexagonal structure.
 4. The imaging device ofclaim 1, wherein the first array and the second array are arranged aspart of a cylindrical structure with the flexible dielectric curvedbetween the first array and the second array.
 5. The imaging device ofclaim 1, wherein the imaging device includes a third array of ultrasonictransducer elements in an arrangement with the first and second arrayssuch that the first and second arrays are side-looking ultrasonic arraysand the third array is a front-looking ultrasonic array.
 6. The imagingdevice of claim 1, wherein the first array and the second array areimager arrays.
 7. The imaging device of claim 1, wherein each ultrasonictransducer element of the first array comprises an integratedmicro-machined ultrasonic transducer.
 8. The imaging device of claim 1,wherein at least one of the first array or the second array isstructured as a two-dimensional ultrasonic transducer array.
 9. Theimaging device of claim 1, wherein the flexible dielectric includes asilicon membrane.
 10. The imaging device of claim 1, wherein the imagingdevice includes signal processing circuitry on a substrate with thefirst array.
 11. The imaging device of claim 10, wherein the substrateis a silicon substrate.
 12. The imaging device of claim 1, wherein theimaging device includes a light source.
 13. The imaging device of claim1, wherein the imaging device includes a CMOS camera.
 14. The imagingdevice of claim 1, wherein the imaging device includes a dome enclosingthe first array, the second array, and the flexible dielectric.
 15. Theimaging device of claim 14, wherein the imaging device includes anantenna within the dome.
 16. The imaging device of claim 1, wherein eachtransducer element comprises a substrate having a doped surface creatinga conducting surface layer, a layer of thermal oxide on the substrate, alayer of silicon nitride on the layer of thermal oxide, a layer ofsilicon dioxide on the layer of silicon nitride, and a layer ofconducting thin film on the layer of silicon dioxide, the layers ofsilicon dioxide and thermal oxide sandwiching the layer of siliconnitride, the layer of conducting thin film separated from the layer ofsilicon nitride by the layer of silicon dioxide.